The rapid resetting of the Ca isotopic signatures of calcite at ambient temperature during its congruent dissolution, precipitation, and at equilibrium

This study provides direct experimental evidence of the resetting of the calcium (Ca) isotope signatures of calcite in the presence of an aqueous fluid during its congruent dissolution, precipitation, and at equilibrium at ambient temperatures over week-long timescales. Batch reactor experiments were performed at 25 °C in aqueous NaCl solutions; air or CO2-gas mixtures were bubbled through this fluid to fix pH. During congruent calcite dissolution, the fluid became enriched in isotopically heavy Ca, and the Ca isotope composition continued to become heavier after the fluid attained bulk chemical equilibrium with the mineral; the δ44/42Ca composition of the fluid was up to 0.8‰ higher than the dissolving calcite at the end of the dissolution experiments. Calcite precipitation was provoked by increasing the reactor fluid pH after chemical equilibrium had been attained via dissolution. Rayleigh isotope fractionation effects were observed immediately after the pH was increased and rapid calcite precipitation occurred. However, isotopic exchange continued after the system chemically equilibrated, eradicating this Rayleigh signal. Taken together, these observations 1) confirm dynamic mineral-fluid equilibrium (i.e. dissolution and precipitation occur at equal, non-zero rates at equilibrium), and 2) indicate that isotopic compositions of calcite can readily equilibrate even when this mineral is in bulk chemical equilibrium with its coexisting fluid. This latter observation suggests the preservation of paleo-environmental isotopic signatures in calcite may require a combination of the isolation of the fluid-mineral system from external chemical input and/or the existence of a yet to be defined calcite dissolution/precipitation inhibition mechanism

The effect of shell secretion rate on Mg / Ca and Sr / Ca ratios in biogenic calcite as observed in a belemnite rostrum

ArticleThis is the final version of the article. Available from European Geosciences Union via the DOI in this record.Isotopic ratios and concentrations of the alkaline earth metals Mg and Sr in biogenic calcite are of great importance as proxies for environmental parameters. In particular, the Mg / Ca ratio as a temperature proxy has had considerable success. It is often hard to determine, however, which parameter ultimately controls the concentration of these elements in calcite. Here, multiple Mg / Ca and Sr / Ca transects through a belemnite rostrum of Passaloteuthis bisulcata (Blainville, 1827) are used to isolate the effect of calcite secretion rate on incorporation of Mg and Sr into the calcite. With increasing calcite secretion rate Mg / Ca ratios decrease and Sr / Ca ratios in the rostrum increase. In the studied specimen this effect is found to be linear for both element ratios over a calcite secretion rate increase of ca. 150 %. Mg / Ca ratios and Sr / Ca ratios show a linear co-variation with increasing relative growth rate, where a 100 % increase in growth rate leads to a (8.1 ± 0.9) % depletion in Mg and a (5.9 ± 0.7) % enrichment in Sr. The magnitude of the calcite secretion rate effect on Mg is (37 ± 4) % greater than that on Sr. These findings are qualitatively confirmed by a geochemical transect through a second rostrum of Passaloteuthis sp. Growth rate effects are well defined in rostra of Passaloteuthis, but only account for a minor part of chemical heterogeneity. Biasing effects on palaeoenvironmental studies can be minimized by informed sampling, whereby the apex and apical line of the rostrum are avoided.Analyses and Philip Pogge von Strandmann were funded by NERC research fellowship grant NE/I020571/2. Clemens Ullmann acknowledges funding from the Leopoldina – German National Academy of Sciences (grant no. LPDS 2014-08). Kate Littler is thanked for comments on an earlier version of this paper. The authors thank the Associate Editor David Gillikin, Adrian Immenhauser and one anonymous reviewer for constructive comments that helped to significantly improve the quality of the manuscript

The Li isotope response to mountain uplift

Silicate weathering is a key process by which CO2 is removed from the atmosphere. It has been proposed that mountain uplift caused an increase in silicate weathering, and led to the long-term Cenozoic cooling trend, although this hypothesis remains controversial. Lithium isotopes are a tracer of silicate weathering processes, which may allow this hypothesis to be tested. Recent studies have demonstrated that the Li isotope ratio in seawater increased during the period of Himalayan uplift (~45 Ma), but the relationship between uplift and the Li isotope ratio of river waters has not been tested. Here we examine Li isotope ratios in rivers draining catchments with variable uplift rates from South Island, New Zealand. A negative trend between δ7Li and uplift shows that areas of rapid uplift have low δ7Li, whereas flatter floodplain areas have high δ7Li. Combined with U activity ratios, the data suggest that primary silicates are transported to floodplains, where δ7Li and (234U/238U) are driven to high values due to preferential uptake of 6Li by secondary minerals, and long fluid-mineral contact times that enrich waters in 234U. In contrast, in mountainous areas, fresh primary mineral surfaces are continuously provided, driving δ7Li and (234U/238U) low. This is the opposite trend to that expected if the increase in Cenozoic δ7Li in the oceans is driven directly by mountain uplift. These data suggests that, rather than weathering of mountain belts, the increase in seawater δ7Li reflects the formation of floodplains and the increased formation of secondary minerals

Assessing bulk carbonates as archives for seawater Li isotope ratios

Silicate weathering is a primary control on the carbon cycle and therefore long-term climate. Tracing silicate weathering in the geological record has been a challenge for decades, with a number of proxies proposed and their limits determined. Recently lithium isotopes in marine carbonates have emerged as a potential tracer. Bulk carbonates are increasingly being used as a Li isotope archive, though with limited tests thus far of the robustness of this approach in the modern ocean. As the bulk composition of marine pelagic carbonates has changed through time and geographically, assessing the fidelity of bulk carbonate as proxy carrier is fundamental. To address the impact of compositional variability in bulk carbonate on Li isotopes, we examine 27 Bahamian aragonitic bulk carbonates and 16 Atlantic largely calcitic core-top sediment samples. Two core-tops only have trace (<10 %) carbonate, and are analysed to test whether carbonates in such sections are still a viable archive. We selectively extract the exchangeable and carbonate fractions from the core-top samples. The exchangeable fraction contains ∼2 % of the total Li and has a fairly constant offset from seawater of 16.5 ± 0.8‰. When leaching silicate-containing carbonates, acetic acid buffered with sodium acetate appears a more robust method of solely attacking carbonates compared to dilute HCl, which may also liberate some silicate-bound Li. Carbonates from samples that do not contain aragonite have the isotopic fractionation of seawater of Δ7Liseawater-calcite = 6.1 ± 1.3‰ (2sd), which is not affected by latitude or the water depth the sample was deposited at. The pure aragonite bulk carbonates from the Bahamas have a fractionation of Δ7Liseawater-aragonite = 9.6 ± 0.6‰. A sediment sample from the Galician coast that mostly consists of quartz is highly offset from seawater by ∼20‰ and also has relatively high Li/Ca ratios. These high values are not due to leaching of silicate material directly (Al/Ca ratios are low). We interpret this addition via cation exchange of Li from silicate during recrystallisation. Overall bulk carbonates from the open ocean are a reliable archive of seawater δ7Li, but care must be taken with carbonate mineralogy and low-carbonate samples. Overall, therefore, any examination of the palaeo-seawater δ7Li record must be reproduced in different global settings (e.g. multiple global cores) before it can be considered robust

Lithium Isotopes: A Tracer of Past and Present Silicate Weathering

Lithium isotopes are a relatively novel tracer of present and past silicate weathering processes. Given that silicate weathering is the primary long-term method by which CO2 is removed from the atmosphere, Li isotope research is going through an exciting phase. We show the weathering processes that fractionate dissolved and sedimentary Li isotope ratios, focusing on weathering intensity and clay formation. We then discuss the carbonate and silicate archive potential of past seawater δ7Li. These archives have been used to examine Li isotope changes across both short and long timescales. The former can demonstrate the rates at which the climate is stabilised from perturbations via weathering, a fundamental piece of the puzzle of the long-term carbon cycle

Modern and Cenozoic records of seawater magnesium from foraminiferal Mg isotopes

Magnesium is an element critically involved in the carbon cycle, because weathering of Ca-Mg silicates removes atmospheric CO2 into rivers, and formation of Ca-Mg carbonates in the oceans removes carbon from the ocean-atmosphere system. Hence the Mg cycle holds the potential to provide valuable insights into Cenozoic climate-system history, and the shift during this time from a greenhouse to icehouse state. We present Mg isotope ratios for the past 40 Myr using planktic foraminifers as an archive. Modern foraminifera, which discriminate against elemental and isotopically heavy Mg during calcification, show no correlation between the Mg isotope composition (δ26Mg, relative to DSM-3) and temperature, Mg / Ca or other parameters such as carbonate saturation (ΔCO3). However, inter-species isotopic differences imply that only well-calibrated single species should be used for reconstruction of past seawater. Seawater δ26Mg inferred from the foraminiferal record decreased from ~0‰ at 15 Ma, to −0.83‰ at the present day, which coincides with increases in seawater lithium and oxygen isotope ratios. It strongly suggests that neither Mg concentrations nor isotope ratios are at steady state in modern oceans, given its ~10 Myr residence time. From these data, we have developed a dynamic box model to understand and constrain changes in Mg sources to the oceans (rivers) and Mg sinks (dolomitisation and hydrothermal alteration). Our estimates of seawater Mg concentrations through time are similar to those independently determined by pore waters and fluid inclusions. Modelling suggests that dolomite formation and the riverine Mg flux are the primary controls on the δ26Mg of seawater, while hydrothermal Mg removal and the δ26Mg of rivers are more minor controls. Using Mg riverine flux and isotope ratios inferred from the 87Sr / 86Sr record, the modelled Mg removal by dolomite formation shows minima in the Oligocene and at the present day (with decreasing trends from 15 Ma), both coinciding with rapid decreases in global temperatures

On the use of Li isotopes as a proxy for water–rock interaction in fractured crystalline rocks: a case study from the Gotthard rail base tunnel

We present Li isotope measurements of groundwater samples collected during drilling of the 57 km long Gotthard rail base tunnel in Switzerland, to explore the use of Li isotope measurements for tracking water–rock interactions in fractured crystalline rocks at temperatures of up to 43 °C. The 17 groundwater samples originate from water-conducting fractures within two specific crystalline rock units, which are characterized by a similar rock mineralogy, but significantly different fluid composition. In particular, the aqueous Li concentrations observed in samples from the two units vary from 1–4 mg/L to 0.01–0.02 mg/L. Whereas δ7Li values from the unit with high Li concentrations are basically constant (δ7Li = 8.5–9.1‰), prominent variations are recorded for the samples from the unit with low Li concentrations (δ7Li = 10–41‰). This observation demonstrates that Li isotope fractionation can be highly sensitive to aqueous Li concentrations. Moreover, δ7Li values from the unit with low Li concentrations correlate well with reaction progress parameters such as pH and [Li]/[Na] ratios, suggesting that δ7Li values are mainly controlled by the residence time of the fracture groundwater. Consequently, 1D reactive transport modeling was performed to simulate mineral reactions and associated Li isotope fractionation along a water-conducting fracture system using the code TOUGHREACT. Modeling results confirm the residence time hypothesis and demonstrate that the absence of δ7Li variation at high Li concentrations can be well explained by limitation of the amount of Li that is incorporated into secondary minerals. Modeling results also suggest that Li uptake by kaolinite forms the key process to cause Li isotope fractionation in the investigated alkaline system (pH >9), and that under slow flow conditions (<10 m/year), this process is associated with a very large Li isotope fractionation factor (ε ≈ −50‰). Moreover, our simulations demonstrate that for simple and well-defined systems with known residence times and low Li concentrations, δ7Li values may help to quantify mineral reaction rates if more thermodynamic data about the temperature-dependent incorporation of Li in secondary minerals as well as corresponding fractionation factors become available in the future. In conclusion, δ7Li values may be a powerful tool to track water–rock interaction in fractured crystalline rocks at temperature higher than those at the Earth’s surface, although their use is restricted to low Li concentrations and well defined flow systems

The Dissolution of Olivine Added to Soil at 4°C: Implications for Enhanced Weathering in Cold Regions

Crushed olivine was added to a soil core to mimic enhanced weathering, and water was continually dripped through for ~6 months. Our experiments were conducted at 4°C, and are compared to previously run identical experiments at 19°C. Olivine dissolution rates in both experiments start out similar, likely due to fines and sharp crystal corners. However, after >100 days of reaction, the dissolution rate at 4°C was two orders of magnitude lower than at 19°C. The accumulation of heavy metals, such as Ni and Cd, was low in both experiments, but soil retention of these elements was proportionally higher at higher temperatures, likely due to enhanced sorption and formation of clays. Overall, this study suggests that olivine dissolution rates in experiments that mimic natural settings are orders of magnitude slower than in normal laboratory experiments, and that enhanced weathering may be a considerably less efficient method of carbon dioxide removal at low climatic temperatures. Both of these conclusions have implications for the application of enhanced weathering as a CO2 removal method

Deep fluid release in warm subduction zones from a breached slab seal

Petrological models and seismic data from subduction zones with geotherms of 7 K km−1 or higher suggest that slabs in these systems dehydrate effectively in the forearc. A large fluid flux is nevertheless released from these slabs at and beyond subarc depth, suggesting that large amounts of H2O can remain slab-bound to much greater depth than expected. We propose that this is due to a transient sealing effect exerted by the subducting lower crust. To test this concept, the petrological and geochemical evolution of such gabbroic crust is investigated through a textural, petrological and Li-chronometric analysis of eclogitized gabbros from an exhumed ultrahigh-pressure terrane. The samples record pristine transitions from dry, rigid gabbro to hydrated eclogite and eclogite mylonite, which occurred when these rocks resided at 90-110 km depth. The observations characterize step-by-step the deformation and overstepped mineral reactions that following the influx of external fluids along a developing network of permeable shear zones. Lithium chronometry indicates that the gabbroic rocks were breached and permeated within a few weeks at a very specific depth within the 90-110 km interval—depths where, in warm subduction zones, large fluid-filled channel system emanate from the slab. The data support a model in which fluids produced in the deserpentinizing slab mantle are trapped at high pore pressure beneath the slab Moho and are ultimately released at subarc depth where the lower crust fails and develops highly permeable fluid vents. The subducting lower crust thus may play an important role in regulating H2O and element budgets, and controlling slab rheology in warm subduction zones